A controller based on Ackermann's method and the other - on the linear quadratic regulator (LQR) approach - were presented. The latter constitutes a challenge for UAV control performance ...Dynamic Programming approach: Here are the following Ackermann equations that would be used to come up with efficient solution. A 2d DP table of size ( (m+1) x (n+1) ) is created for storing the result of each sub-problem. Following are the steps demonstrated to fill up the table. Filled using A ( 0, n ) = n + 1 The very next method is to …Ackermann(2,4) = 11. Practical application of Ackermann's function is determining compiler recursion performance. Solve. Solution Stats. 36.61% Correct | 63.39% Incorrect. 224 Solutions; 69 Solvers; Last Solution submitted on Dec 12, 2023 Last 200 Solutions. Problem Comments. 2 Comments.It is referred to as kinematics because Ackermann's principle of steering doesn’t get influenced by any external forces. It involves only the relative motion between force links and doesn’t involve the study of the effect of forces. The Ackermann steering geometry is designed in such a way that the two front wheels are always aligned ...Choose the desired pole location, then compute the gain K required to achieve those locations Ackermann’s formula for SISO systems (Matlab’s ‘acker’) Matlab’s ‘place’ for MIMO systems! !Ackermann’s Formula • Thepreviousoutlinedadesignprocedureandshowedhowtodoit byhandforsecond-ordersystems. – …Ackerman Steering. An elegant and simple mechanism to approximate ideal steering was patented in England in 1818 by Rudolph Ackerman, and though it is named after him, the actual inventor was a German carriage builder called Georg Lankensperger who designed it two years earlier.poles, Ackermann’s formula, feedback invariants, deadbeat control, reviving the Brunovski structure, Hessenberg form. Contents 1. Introduction 2. Separation of state observation and state feedback 3. The single-input case 3.1 Ackermann’s formula 3.2 Numerically stable calculation via Hessenberg form 4. The multi-input case 4.1 Non-uniqueness ; ; Ackermann function for Motorola 68000 under AmigaOs 2+ by Thorham ; ; Set stack space to 60000 for m = 3, n = 5. ; ; The program will print the ackermann values for the range m = 0..3, n = 0..5 ; _LVOOpenLibrary equ -552 _LVOCloseLibrary equ -414 _LVOVPrintf equ -954 m equ 3 ; Nr of iterations for the main loop. n equ 5 ; Do NOT set …Ackermann function. In computability theory, the Ackermann function, named after Wilhelm Ackermann, is one of the simplest [1] and earliest-discovered examples of a total computable function that is not primitive recursive. All primitive recursive functions are total and computable, but the Ackermann function illustrates that not all total ... Sep 26, 2022 · Dynamic Programming approach: Here are the following Ackermann equations that would be used to come up with efficient solution. A 2d DP table of size ( (m+1) x (n+1) ) is created for storing the result of each sub-problem. Following are the steps demonstrated to fill up the table. Filled using A ( 0, n ) = n + 1 The very next method is to fill ... This page is based on the copyrighted Wikipedia article "Ackermann%27s_formula" ; it is used under the Creative Commons Attribution-ShareAlike 3.0 Unported License. You may redistribute it, verbatim or modified, providing that you comply with the terms of the CC-BY-SA. abcdef.wiki is not affiliated with the Wikimedia FoundationWilhelm Friedrich Ackermann (/ ˈ æ k ər m ə n /; German: [ˈakɐˌman]; 29 March 1896 – 24 December 1962) was a German mathematician and logician best known for his work in mathematical logic and the Ackermann function, an important example in …Hàm Ackermann đôi khi còn được gọi là hàm Ackermann-Peter. Lịch sử [ sửa | sửa mã nguồn ] Hàm Ackermenn được trình bày lần đầu tiên trong một cuốn sách về logic (mà nhà toán học David Hilbert là đồng tác giả) tựa đề Đức ngữ là Grundzuege der Theoretischen Logik (dịch nghĩa ...Apr 6, 2022 · Subject - Control System 2Video Name - Concept of pole placement for controller design via Ackerman methodChapter - Control Systems State Space AnalysisFacul... 326 Marius Costandin, Petru Dobra and Bogdan Gavrea 2. The novel proof for Ackermann’s formula Theorem 2.1 (Ackermann). Let X_ = AX+Bube a linear time invariant dynamicalUsing a corner radius equal to their wheelbase is common. The percentage of Ackermann would be equal to the percentage from 100% Ackermann that your particular steering geometry exhibits. For example, you use an inside wheel steering angle of 15 degrees and the outside wheel is at 12 degrees. If 100% Ackermann is when the outside wheel is at …Subject - Control System 2Video Name - Concept of pole placement for controller design via Ackerman methodChapter - Control Systems State Space AnalysisFacul...Feb 22, 2019 · Ackermann Function. A simple Matlab function to calculate the Ackermann function. The Ackerman function, developed by the mathematician Willhelm Ackermann, impresses with its extremely fast growth and has many more fascinating features. With this simple code, the Ackermann function can be easily used in Matlab. The Kinematic Steering block implements a steering model to determine the left and right wheel angles for Ackerman, rack-and-pinion, and parallel steering mechanisms. The block uses the vehicle coordinate system. To specify the steering type, use the Type parameter. Ideal Ackerman steering, adjusted by percentage Ackerman.Sat Jan 04, 2014 6:22 pm. The first picture is anti ackerman. The second is pro ackerman. There is loads of information on this if you both to look. BTW, anti ackerman seems to be pretty common in F1 at Monaco. I don't know the particulars as to why, but its usually a tyre driven design choice.Jan 18, 2024 · The Ackermann function is the simplest example of a well-defined total function which is computable but not primitive recursive, providing a counterexample to the belief in the early 1900s that every computable function was also primitive recursive (Dötzel 1991). It grows faster than an exponential function, or even a multiple exponential function. The Ackermann function A(x,y) is defined for ... optimized by using mathematical equations for ackermann mechanism for different inner wheel angles also we get ackermann percentage from this geometrical equation. To design the vehicle steering (four wheeler), this mathematical model can be applied to rear wheel steering also. REFERENCES 1. Theory of Machines, Khurmi Gupta. 2.optimized by using mathematical equations for ackermann mechanism for different inner wheel angles also we get ackermann percentage from this geometrical equation. To design the vehicle steering (four wheeler), this mathematical model can be applied to rear wheel steering also. REFERENCES 1. Theory of Machines, Khurmi Gupta. 2.•Ackermann’s Formula •Using Transformation Matrix Q. Observer Gain Matrix •Direct Substitution Method Oct 30, 2008 · SVFB Pole Placement and Ackermann's Formula We would like to choose the feedback gain K so that the closed-loop characteristic polynomial Δc (s) =sI −Ac =sI −(A−BK) has prescribed roots. This is called the POLE-PLACEMENT problem. An important theorem says that the poles may be placed arbitrarily as desired iff (A,B) is reachable. Ackermann Design for Observers When there is only one output so thatp =1, one may use Ackermann's formula. Thus, select the desired observer polynomial ∆ oD (s) and replace (A,B) in K e U 1 (A) = n ∆ oD −, by (AT ,CT ), then set L = KT. We can manipulate this equation into its dual form using matrix transposition to write ( ) 1 (T) oD …In the first two publications (Valasek and Olgac, 1995a, Automatica, 31(11) 1605–1617 and 1995b IEE Control Theory Appl. Proc 142 (5), 451–458) the extension of Ackermann’s formula to time ...Ackermann’s formula still works. Note that eig(A−LC) = eig(A−LC) T= eig(A −C LT), and this is exactly the same as the state feedback pole placement problem: A−BK. Ackermann’s formula for L Select pole positions for the error: η1,η2,···,ηn. Specify these as the roots of a polynomial, γo(z) = (z −η1)(z −η2)···(z −ηn). Ackermann function Peter Mayr Computability Theory, February 15, 2021. Question Primitive recursive functions are computable. What about the converse? We’ll see that some functions grow too fast to be primitive recursive. Knuth’s up arrow notation. a "n b is de ned by a "b := a|{z a} b a ""b := a a |{z} bThis page is based on the copyrighted Wikipedia article "Ackermann%27s_formula" ; it is used under the Creative Commons Attribution-ShareAlike 3.0 Unported License. You may redistribute it, verbatim or modified, providing that you comply with the terms of the CC-BY-SA. abcdef.wiki is not affiliated with the Wikimedia FoundationThe Ackermann function, due to its definition in terms of extremely deep recursion, can be used as a benchmark of a compiler's ability to optimize recursion. The first use of Ackermann's function in this way was by Yngve Sundblad, The Ackermann function. A Theoretical, computational and formula manipulative study. (BIT 11 (1971), 107119).Jan 1, 2023 · The Ackermann's formula of pole placement for controllable linear time invariant (LTI) systems is extended to multi input LTI systems by employing generalized inversion of the system's controllability matrix instead of square inversion in the procedure of deriving the formula. The nullspace of the controllability matrix is affinely and ... Oct 30, 2008 · SVFB Pole Placement and Ackermann's Formula We would like to choose the feedback gain K so that the closed-loop characteristic polynomial Δc (s) =sI −Ac =sI −(A−BK) has prescribed roots. This is called the POLE-PLACEMENT problem. An important theorem says that the poles may be placed arbitrarily as desired iff (A,B) is reachable. アッカーマン関数 （アッカーマンかんすう、 英: Ackermann function 、 独: Ackermannfunktion ）とは、非負 整数 m と n に対し、. によって定義される 関数 のことである。. [1] 与える数が大きくなると爆発的に 計算量 が大きくなるという特徴があり、性能測定などに ... Ackermann's original function is defined as follows: \begin {equation*} \varphi ( a , b , 0 ) = \alpha + b, \end {equation*} \begin {equation*} \varphi ( a , 0,1 ) = 0 , \varphi …Following are the steps to be followed in this particular method. Check the state controllability of the system. 2. Define the state feedback gain matrix as. – And equating equation. Consider the regulator system shown in following figure. The plant is given by. The system uses the state feedback control u=-Kx.Pole Placement using Ackermann’s Formula. The Ackermann’s formula is, likewise, a simple expression to compute the state feedback controller gains for pole …The Ackermann Function A(m,n) m=0. A(m,n)=n+1Ackermann and coworkers have investigated a palladium acetate-catalyzed domino reaction sequence in the presence of tricyclohexylphosphine (under two alternative base and solvent conditions) between anilines or diarylamines (417) and aryl-1,2-dihalides (418).The sequence consisted of an intermolecular N-arylation and an intramolecular …optimized by using mathematical equations for ackermann mechanism for different inner wheel angles also we get ackermann percentage from this geometrical equation. To design the vehicle steering (four wheeler), this mathematical model can be applied to rear wheel steering also. REFERENCES 1. Theory of Machines, Khurmi Gupta. 2. Pole Placement using Ackermann’s Formula. The Ackermann’s formula is, likewise, a simple expression to compute the state feedback controller gains for pole …The Ackermann formula is a method of designing control systems to solve the pole-assignment problem for invariant time systems. One of the main problems in the design of control systems is the creation of controllers that will change the dynamics of a system by changing the eigenvalues of the matrix that represents the dynamics of the …Jan 1, 2023 · The Ackermann's formula of pole placement for controllable linear time invariant (LTI) systems is extended to multi input LTI systems by employing generalized inversion of the system's controllability matrix instead of square inversion in the procedure of deriving the formula. The nullspace of the controllability matrix is affinely and ... The matrix Cayley-Hamilton theorem is first derived to show that Ackermann's formula for the pole-placement problem of SISO systems can be extended to the case of a class of MIMO systems. Moreover, the extended Ackermann formula newly developed by the authors is employed for fast determination of the desired feedback gain …Jan 18, 2024 · The Ackermann function is the simplest example of a well-defined total function which is computable but not primitive recursive, providing a counterexample to the belief in the early 1900s that every computable function was also primitive recursive (Dötzel 1991). It grows faster than an exponential function, or even a multiple exponential function. The Ackermann function A(x,y) is defined for ... Sliding mode control design based on Ackermann's formula. Jürgen Ackermann, Vadim I. Utkin. Sliding mode control design based on Ackermann's formula. IEEE Trans. Automat. Contr., 43(2): 234-237, 1998.poles, Ackermann’s formula, feedback invariants, deadbeat control, reviving the Brunovski structure, Hessenberg form. Contents 1. Introduction 2. Separation of state observation and state feedback 3. The single-input case 3.1 Ackermann’s formula 3.2 Numerically stable calculation via Hessenberg form 4. The multi-input case 4.1 Non-uniqueness This formula for the state feedback matrix is known as “Ackermann’s formula.” The Matlab commands ackerand placefind the required K for a given (A;B) and a given set of required closed-loop eigenvalues. 5.3 Tracking in state-space systems Tracking external references in the state-space configuation is not much different In the first two publications (Valasek and Olgac, 1995a, Automatica, 31(11) 1605–1617 and 1995b IEE Control Theory Appl. Proc 142 (5), 451–458) the extension of Ackermann’s formula to time ...$\begingroup$ Oh, sorry! Well take my heading vector <259.9359375, 260.6359375, 261.0359375> and calculate the steering angle using a 5 meter wheelbase and a 3 meter track width, we get <81.84434488 81.66116341 81.43259016>.About Press Copyright Contact us Creators Advertise Developers Terms Privacy Policy & Safety How YouTube works Test new features NFL Sunday Ticket …$\begingroup$ Oh, sorry! Well take my heading vector <259.9359375, 260.6359375, 261.0359375> and calculate the steering angle using a 5 meter wheelbase and a 3 meter track width, we get <81.84434488 81.66116341 81.43259016>.See also inverse Ackermann function. Note: Many people have defined other similar functions which are not simply a restating of this one. In 1928, Wilhelm Ackermann observed that A(x,y,z), the z-fold iterated exponentiation of x with y, is a recursive function that is not primitive recursive. A(x,y,z) was simplified to a function of 2 variables ...This paper presents a novel proof for the well known Ackermann's formula, related to pole placement in linear time invariant systems. The proof uses a lemma [3], concerning rank one updates for ...The Ackermann function is defined for integer and by (1) Special values for integer include Expressions of the latter form are sometimes called power towers. follows …ackermann’s formula for design using pole placement [5–7] In addition to the method of matching the coefficients of the desired characteristic equation with the coefficients of det ( s I − P h ) as given by Eq (8.19) , Ackermann has developed a competing method. 3 MODERN CONTROL-SYSTEM DESIGN USING STATE-SPACE, POLE PLACEMENT, ACKERMANN'S FORMULA, ESTIMATION, ROBUST CONTROL, AND H ∞ TECHNIQUES 3.1. INTRODUCTION. State-space analysis was introduced in Chapter 1, and has been used in parallel with the classical frequency-domain analyses techniques presented in …; ; Ackermann function for Motorola 68000 under AmigaOs 2+ by Thorham ; ; Set stack space to 60000 for m = 3, n = 5. ; ; The program will print the ackermann values for the range m = 0..3, n = 0..5 ; _LVOOpenLibrary equ -552 _LVOCloseLibrary equ -414 _LVOVPrintf equ -954 m equ 3 ; Nr of iterations for the main loop. n equ 5 ; Do NOT set …Ackermann Design for Observers When there is only one output so thatp =1, one may use Ackermann's formula. Thus, select the desired observer polynomial ∆ oD (s) and replace (A,B) in K e U 1 (A) = n ∆ oD −, by (AT ,CT ), then set L = KT. We can manipulate this equation into its dual form using matrix transposition to write ( ) 1 (T) oD …Part 4 Unit 5: Pole PlacementJan 11, 2022 · In the second method (Switching surface design via Ackermann’s formula) which proposes a scalar sliding mode control design depends on the desired eigenvalues and the controllability matrix to achieve the desired sliding mode control performance with respect to its flexibility of solution. Request PDF | On Aug 18, 2008, Gopal Jee and others published Generalization of Ackermann's Formula for State Feedback of Multi-Input Systems | Find, read and cite all the research you need on ...poles, Ackermann’s formula, feedback invariants, deadbeat control, reviving the Brunovski structure, Hessenberg form. Contents 1. Introduction 2. Separation of state observation and state feedback 3. The single-input case 3.1 Ackermann’s formula 3.2 Numerically stable calculation via Hessenberg form 4. The multi-input case 4.1 Non-uniqueness following Ackermann formula: kT =−q(R+)−1p(A) which can be used only if matrix R+ is squared and invertible, that is only if the system is completely reachable and has only one input. ZanasiRoberto-SystemTheory. A.A.2015/2016. Title: …Sep 1, 2015 · Ackermann's formula (volume = 0.6 × stone surface 1.27), established with the help of computer software 15 and proposed in the recommendations of the EAU until 2009. 13, 17, 18. The Ackermann's formula is advantageous as it can integrate the surface in the calculations (Surface = L × W × π × 0.25). However, in practice, we often only know ... To write the equation representing a fixed value of n as 4, we need some other notation, since the time complexity is greater than exponential.. Hyperoperations. The time complexity for Ackermann ...The sliding mode control methods are developed to design systems which have the desired dynamic behavior and are robust with respect to perturbations. It is shown that the discontinuity plane for sliding mode control may be found in an explicit form using Ackermann's formula. Two design procedures are derived. First, static controllers are designed to enforce sliding modes with the desired ... #Pole_Placement #Ackerman's_Formula #Control_System. About Press Press아커만 함수. 계산 가능성 이론 에서, 빌헬름 아커만 의 이름을 딴 아커만 함수 (Ackermann函數, 영어: Ackermann function )는 원시 재귀 함수 가 아닌 전역적인 재귀 함수 (계산가능 함수)의 가장 간단한 예시로, 가장 먼저 발견된 것이기도 하다. 모든 원시 재귀 함수는 ... ٦. Note that if the system is not completely controllable, matrix K cannot be determined. (No solution exists.) ٧. The system uses the state feedback control u=–Kx. Let us choose the desired closed-loop poles at. Determine the state feedback gain matrix K. ٨. By defining the desired state feedback gain matrix K as. Following are the steps to be followed in this particular method. Check the state controllability of the system. 2. Define the state feedback gain matrix as. – And equating equation. Consider the regulator system shown in following figure. The plant is given by. The system uses the state feedback control u=-Kx.This formula for the state feedback matrix is known as “Ackermann’s formula.” The Matlab commands ackerand placefind the required K for a given (A;B) and a given set of required closed-loop eigenvalues. 5.3 Tracking in state-space systems Tracking external references in the state-space configuation is not much different Choose the desired pole location, then compute the gain K required to achieve those locations Ackermann’s formula for SISO systems (Matlab’s ‘acker’) Matlab’s ‘place’ for MIMO systems! !Topic: Controller Design using Ackermann’s FormulaAssignment1.Write Ackerman's Formula2.Define:Eigen Value3.List the properties of Eigen Value4.How to fine i...The Ackermann function, due to its definition in terms of extremely deep recursion, can be used as a benchmark of a compiler's ability to optimize recursion. The first use of Ackermann's function in this way was by Yngve Sundblad, The Ackermann function. A Theoretical, computational and formula manipulative study. (BIT 11 (1971), 107119). Ackermann Steering refers to the geometric configuration that allows both front wheels to be steered at the appropriate angle to avoid tyre sliding. For a given turn radius R, wheelbase L, and track width T, …Choose the desired pole location, then compute the gain K required to achieve those locations Ackermann’s formula for SISO systems (Matlab’s ‘acker’) Matlab’s ‘place’ for MIMO systems! !#Pole_Placement #Ackerman's_Formula #Control_System. About Press PressThe celebrated method of Ackermann for eigenvalue assignment of single-input controllable systems is revisited in this paper, contributing an elegant proof. The new proof facilitates a compact formula which consequently permits an extension of the method to what we call incomplete assignment of eigenvalues. The inability of Ackermann’s …$\begingroup$ Oh, sorry! Well take my heading vector <259.9359375, 260.6359375, 261.0359375> and calculate the steering angle using a 5 meter wheelbase and a 3 meter track width, we get <81.84434488 81.66116341 81.43259016>.Ackermann function. In computability theory, the Ackermann function, named after Wilhelm Ackermann, is one of the simplest [1] and earliest-discovered examples of a total computable function that is not primitive recursive. All primitive recursive functions are total and computable, but the Ackermann function illustrates that not all total ... In control theory, Ackermann's formula is a control system design method for solving the pole allocation problem for invariant-time systems by Jürgen Ackermann. One of the primary problems in control system design is the creation of controllers that will change the dynamics of a system by … See moreGraham's number was used by Graham in conversations with popular science writer Martin Gardner as a simplified explanation of the upper bounds of the problem he was working on. In 1977, Gardner described the number in Scientific American, introducing it to the general public.At the time of its introduction, it was the largest specific positive integer ever to …A multi-variable function from the natural numbers to the natural numbers with a very fast rate of growth. In 1928, W. Ackermann , in connection with some problems that his PhD supervisor, D. Hilbert, was investigating, gave an example of a recursive (i.e., computable) function that is not primitive recursive.(A primitive recursive function is one …326 Marius Costandin, Petru Dobra and Bogdan Gavrea 2. The novel proof for Ackermann’s formula Theorem 2.1 (Ackermann). Let X_ = AX+Bube a linear time invariant dynamical
Sliding mode control of yaw movement based on Ackermann's formula Abstract: A ship in open sea is a very complex dynamic system. It is affected by three types of perturbations: hydrodynamic perturbations induced by the ship movements, external perturbations produced by wind, waves, and sea currents, and those produced by the control systems …. Brand new cd skipping
A comprehensive study for pole placement of DC motor is studied using different state feedback control techniques. It also compares the control parameters perfoAckermann’s Function George Tourlakis February 18, 2008 1 What The Ackermann function was proposed, naturally, by Ackermann. The version here is a simplification offered by Robert Ritchie. What the function does is to provide us with an example of a number-theoretic intuitively computable, total function that is not in PR.A comprehensive study for pole placement of DC motor is studied using different state feedback control techniques. It also compares the control parameters perfoAckermann’s formula still works. Note that eig(A−LC) = eig(A−LC) T= eig(A −C LT), and this is exactly the same as the state feedback pole placement problem: A−BK. Ackermann’s formula for L Select pole positions for the error: η1,η2,···,ηn. Specify these as the roots of a polynomial, γo(z) = (z −η1)(z −η2)···(z −ηn).Apr 27, 2023 · Pole placement can be done using different methods, such as root locus, state feedback, or Ackermann's formula. Add your perspective Help others by sharing more (125 characters min.) Cancel Undefined behaviour. Unfortunately, your code shows undefined behaviour due to access on an uninitialized value and out-of-bounds access. The simplest test that shows this behaviour is m = 1, n = 0.This indicates only two iterations of the outer loop and one iteration of the inner loop and thus is easier to analyze:This paper presents a novel proof for the well known Ackermann's formula, related to pole placement in linear time invariant systems. The proof uses a lemma [3], concerning rank one updates for ...The Ackermann steering geometry is a geometric arrangement of linkages in the steering of a car or other vehicle designed to solve the problem of wheels on the inside and outside of a turn needing to trace out circles of different radii . It was invented by the German carriage builder Georg Lankensperger in Munich in 1816, then patented by his ... following Ackermann formula: kT =−q(R+)−1p(A) which can be used only if matrix R+ is squared and invertible, that is only if the system is completely reachable and has only one input. ZanasiRoberto-SystemTheory. A.A.2015/2016. Title: …Choose the desired pole location, then compute the gain K required to achieve those locations Ackermann’s formula for SISO systems (Matlab’s ‘acker’) Matlab’s ‘place’ for MIMO systems! !A controller based on Ackermann's method and the other - on the linear quadratic regulator (LQR) approach - were presented. The latter constitutes a challenge for UAV control performance ...This widget simply compute the two input Ackermann–Péter function, a function which gives amazingly large numbers for very small input values. Get the free "Ackermann function" …Ackermann function. This widget simply compute the two input Ackermann–Péter function, a function which gives amazingly large numbers for very small input values. Get the free "Ackermann function" widget for your website, blog, Wordpress, Blogger, or iGoogle. Find more Computational Sciences widgets in Wolfram|Alpha. poles, Ackermann’s formula, feedback invariants, deadbeat control, reviving the Brunovski structure, Hessenberg form. Contents 1. Introduction 2. Separation of state observation and state feedback 3. The single-input case 3.1 Ackermann’s formula 3.2 Numerically stable calculation via Hessenberg form 4. The multi-input case 4.1 Non-uniqueness Looking at the Wikipedia page, there's the table of values for small function inputs. I understand how the values are calculated by looking at the table, and how it's easy to see that 5,13,29,61,125 is $2^{n+3}-3$, but how does one go about calculating this "iterative" formula without pattern identification?Electrical Engineering questions and answers. Design a Luenberger observer using Ackermann’s formula assuming that the output θa (t) is the only measurement. Place the observer eigenvalues at λ = −60 ± j3. Question: Design a Luenberger observer using Ackermann’s formula assuming that the output θa (t) is the only measurement. .